High Lockout
F.01 on Vaillant ecoTEC Plus 836

Your boiler has stopped working because it cannot accurately measure the temperature of the water returning to the unit.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Return temperature sensor interruption

What causes this fault?

This fault occurs when the boiler's control board loses communication with the sensor that monitors water temperature coming back from your radiators. The most common reason is that the sensor has developed an internal electrical fault or the connecting wires have become loose or damaged. Because the boiler can no longer track these temperatures safely, it shuts down to prevent overheating.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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